The unidirectional influx of amino acids into the brush border epithelium of chicken jejunum and ileum is being studied in tissues mounted in a chamber so that only the mucosa is exposed to tracer. Absorption kinetics are being followed over a wide concentration range of substrate and analysis is being made for multiple limbed kinetic plots. Specificity of substrate influx is being examined at a variety of substrate concentration extremes. The mechanisms of action of Na ions and theophylline to restore or maintain influx under conditions of Na ions depletion are being examined. In addition, the effects of ouabain, dinitrophenol, fluoride ion, calcium ion, PCMBS, hormones and various effectors of cyclic AMP levels on influx are bein assessed. The overall objective of the work is to evaluate the fundamental mechanisms which operate in transport of amino acids across the brush border and to determine how these mechanisms are controlled by Na ion and cyclic AMP.